1. Field of the Invention
The present invention relates to the holding of a substrate, such as a semiconductor wafer, in a vacuum processing chamber. More particularly, the invention relates to a method and apparatus for improving chucking reproducibility of substrates on electrostatic chucks.
2. Description of the Related Art
Electrostatic chucks are used to hold individual substrates on a pedestal in semiconductor processing systems. One example of an electrostatic chuck is described in co-assigned U.S. Pat. No. 5,315,473 which is incorporated herein by reference. An electrostatic chuck typically includes at least a dielectric layer and an electrode, which may be located on a chamber pedestal or formed as an integral part of the pedestal. A substrate is placed in contact with the dielectric layer, and a direct current voltage is placed on the electrode to create the electrostatic attractive force to grip the substrate. An electrostatic chuck is particularly useful in vacuum processing environments where the maximum differential pressure which can be established between the low pressure chamber environment and the surface of the pedestal is insufficient to firmly grip the substrate or where mechanical clamping of the substrate is undesirable.
Although an electrostatic chuck may be formed from as little as a single dielectric layer and an electrode, a more typical working configuration is a thin laminate member which is supported on a chamber pedestal to receive and support the substrate. The laminate member preferably includes an electrode core, preferably a thin copper member such as a mesh, which is sandwiched between upper and lower dielectric layers of an organic material such as polyimide. An adhesive such as polyamide may be used to attach the polyimide layers to the electrode core. The lower dielectric layer of the laminate member is attached directly to the upper surface of the pedestal, usually with an adhesive such as polyamide, and the upper dielectric layer forms a planar surface on which the substrate is received. To supply the high voltage potential to the electrode, a strap, formed as an integral extension of the laminate member, extends around the edge of the pedestal and connects to a high voltage connector on the underside of the pedestal.
The use of organic materials as the dielectric layer of the electrostatic chuck creates an inherent limitation in the useful life of the chuck, because organic materials, including polyimides, have a relatively low tolerance for many process gases and plasmas, particularly to oxygen and oxygen-based plasmas. Although the majority of the surface area of the dielectric portions of the dielectric chuck are protected from the plasma by the substrate, the dielectric materials usually gets exposed to the plasma over its useful life.
Repetitive plasma process and clean cycles eventually erodes the dielectric layer to the point where arcing may occur between the electrode and the plasma, which destroys the effectiveness of the electrostatic chuck. Dielectric materials other than those particularly mentioned above, may also be adversely affected by exposure to process and cleaning gases and plasmas. For example, dielectrics such as quartz and silicon dioxide are eroded by CHF3 and CF4 plasma environments. Likewise, chuck configurations other than those using the laminate member construction may be adversely affected by process and cleaning gases and plasmas, such that the dielectric layers thereof will be eroded to the point where an arc will form between the electrode and the plasma.
Another problem which occurs with electrostatic chucks is that a charge buildup on the surface of the chuck can occur. As a result, the griping force on the substrate is not instantaneously removed once the bias is removed. In some cases, this is a problem since a significant remnant of this force can remain for as long as 30 to 60 seconds after removal of the clamping voltage. The waiting period required for this charge to leak off before the wafer can be removed decreases the throughput of the system. If excessive force is applied to lift a wafer while the remnant charge force is still present, the wafer can be damaged.
One solution to the dechucking problem has been to dope the ceramic surface of the chuck with conductive impurities to increase the surface conductivity of the chuck. Doping of the surface enables a residual charge to move more easily between the chucking electrode and the wafer interface. This solution makes the surface condition of the chuck critical to the chucking force which is applied to the wafer. However, the surface condition and properties of the chuck can be substantially changed by exposure to process and cleaning plasmas used in a typical system chamber. This is a further problem in that deterioration of the dielectric layer results in premature release of wafers.
There remains a need for a technique for improving the reliability of chucking substrates on electrostatic chucks which eliminates the problems encountered as a result of deterioration of the dielectric layer on electrostatic chucks.